1. Technical Field
The present invention relates generally to thermal imaging systems, and in particular to a system for digitally correcting any channel-to-channel voltage level variations present in a scanning thermal imaging system, thereby ensuring a high quality output video signal.
2. Discussion
Scanning thermal imaging systems are used in a variety of applications, including surveillance systems and target detection/recognition systems. Such systems typically incorporate a telescopic lens assembly coupled to a scanner. The scanner scans energy from a scene through an imager lens assembly onto a detector array having a plurality of photoelectrically responsive detector elements perpendicular to the scan direction. Each of these detector elements provides an electric signal proportional to the flux of infrared light on the particular detector element. Electric signals generated from the detector elements are subsequently processed by system sensor electronics to create an image that is displayed on a system output device. To improve sensitivity, some of these systems incorporate detectors parallel to the scan direction. The output of these detectors are delayed in time from each other such that, ideally, the scanned image is output simultaneously on all of the parallel detectors. The delayed outputs are then summed (integrated). This process is referred to as time delay and integrate (TDI).
In the above-mentioned thermal imaging system detector arrays, it is essential that the multiplexed detector array channels have an associated voltage gain and voltage level that is uniform with all other channels to ensure the quality of the signal that is input into the system electronics for processing. However, each channel may have an associated DC offset variation that causes it to differ from the other channel DC offsets.
Many Presently implemented systems for correcting channel-to-channel non-uniformity require a thermal reference source in the optical path. All of the detectors must be able to view this thermal reference source at a time that the detectors are not viewing the scene. This requirement adds cost and complexity to the opto-mechanical system. To provide optimum correction, the thermal reference source must be at the scene temperature; therefore, it must be able to be cooled and heated by additional electronics that add cost and complexity to the system. Since the scene can have many different temperatures, the correction will only be valid around the temperature of the thermal reference source or multiple sources are required with additional cost and complexity. What is needed, then, is a non-uniformity correction system that uses only scene information to determine the correction values. Moreover, for many applications, this scene based non-uniformity correction must work in the presence of scene motion.